1. Field of the Invention
This invention relates to implantable joint replacement prostheses and, in particular, to a prosthesis having a bearing surface which is orientable. In its preferred embodiments, the prosthesis serves as the tibial component of an artificial knee joint.
2. Description of the Prior Art
Hartmann, German Patent Publication No. 2,154,338, discloses a hinged artificial knee joint having a tibial shaft 2 which includes a tapered bore 3' which receives conical pin 3 of hinge support 1. By means of the tapered bore and the conical pin, the hinge support can be rotated at the time of implantation for "correction" of the alignment of the tibial and femoral components of the joint. A screw 4 which engages a threaded borehole 11' in tibial shaft 2 is used to fix the hinge support within the tibial shaft.
Contemporary artificial knee joints commonly include a bearing element composed of a deformable material, e.g., a plastic such as ultra high molecular weight polyethylene. Such elements provide bearing function generally accepted in the art.
The construction shown in the Hartmann reference contains no provision for the use of such a bearing element composed of a deformable material. In particular, Hartmann's joint requires that hinge support 1 and tibial shaft 2 be composed of non-deformable materials, such as metals, so that these components can be held together in a fixed relation by screw 4. Prostheses having all metal constructions currently have little use in the art.
A tibial prosthesis for a knee joint which has been sold by Joint Medical Products Corporation, the assignee of this application, is shown in FIG. 7. As shown therein, this prosthesis includes: 1) tibial fixation member 100 composed of metal and having an internal conical bore 102; 2) tibial bearing member 104 composed of metal and having projecting conical shaft 106 for locking engagement with bore 102; 3) cap 110 for sealing the distal end of fixation member 100; 4) screw 108 for holding cap 110 and members 100 and 104 together; and 5) tibial plateau bearing member 112 composed of ultra high molecular weight polyethylene and having shaft 114 which is received in internal bore 116 of tibial bearing member 104. Conical bore 102 and conical shaft 106 form a self-locking taper. Because of the clearance between them, conical bore 116 and conical shaft 114 do not form a locking taper so that tibial plateau bearing member 112 is free to rotate on tibial bearing member 104.
Although this commercial construction has worked well in practice, it does not provide a fixed relationship between tibial plateau bearing member 112 and fixation member 100 which is desired for some patients.